Elastomeric compositions in which styrene-butadiene rubber, hereinafter SBR, and a thermoplastic olefin resin such as polypropylene are combined have resulted in thermoplastic compositions. These materials have been proposed for use in some applications where thermoplastic elastomers would be of advantage.
However, many of the products produced by the dynamic vulcanization of SBR in the presence of materials such as polypropylene have not resulted in products which are suitable form some applications, for example, in the pharmaceutical industry. These materials lack cleanliness, high temperature stability or autoclave set.
Because of extremely high quality control and concern for the integrity of the product contained in pharmaceutical containers, thermoplastic elastomers have been slow to be accepted as candidates for container closures, vial stoppers, dropper bulbs and the like. The product must have a high tensile strength and set resistance properties, be soft and thermally stable, and must be easy to process. Of particular need is a material which has a low autoclave set which would be of value as a candidate for pharmaceutical packaging. Until the present time, a method for making such a material has not been developed which has all the many required properties for use successfully in the pharmaceutical industry.
U.S. Pat. No. 4,104,210 and U.S. Pat. No. 4,271,049, both describe two component systems in which high unsaturation diene rubbers and thermoplastic olefins are blended. Two component systems are shown in which SBR is dispersed in polypropylene, where polypropylene is the continuous phase.
U.S. Pat. No. 4,197,377 describes a thermoplastic elastomer blend which about 10 to about 48 parts of a polyolefin such as polypropylene is employed, from 90 to about 10 parts by weight of SBR and a compound selected from triblock copolymers, diblock copolymers and polystyrene. The diblock copolymer is a styrene-butadiene copolymer and the triblock copolymer is a strene-butadiene copolymer. The amount of this third component ranges from about 0.1 to about 80 parts by weight.
U.S. Pat. No. 4,250,273 describes a blend of uncured or partially cured mixtures of SBR, 1-olefine polymers or copolymers and highly saturated elastomers. One example of the highly saturated elastomer is polyisobutylene. The tri-blend of materials generally has the 1-olefin polymer or copolymer and the SBR rubber in the continuous phase.
U.S. Pat. No. 4,340,684 describes thermoplastic elastomeric blends of 1-olefin polymers, SBR rubbers and highly saturated elastomers and is a divisional of the previously mentioned U.S. Pat. No. 4,250,273. Similarly, U.S. Pat. No. 4,350,795 is a divisional of that same patent.
All three of these patents relate to tri-blends in which the 1-olefin polymer or copolymer and the SBR are in the continuous phase. U.S. Pat. No. 4,385,142 is related to these previously described patents, but further include from about 5 to about 50 parts by weight of bitumen.
None of the prior art formulations which are described in those patents have the appropriate high tensile strength and set resistance properties that are desired while also being soft and thermally stable. Methods in which SBR is dynamically vulcanized in the presence of polypropylene and other polyolefins have not resulted in thermoplastic elastomers would be of advantage. None of these method are capable of providing the ideal composition for use in the pharmaceutical industry.
Accordingly, it is an object of this invention to provide a method of making a thermoplastic elastomer composition which is suitable for use as a product in the pharmaceutical industry and which is useful in other application where high tensile strength, low compression set, and thermally stable soft processable materials are desirable.
Other objects will appear hereinafter.